In this century, the shortage of fresh water may rival or surpass the shortage of energy as a global concern for humanity, and these two challenges are inexorably linked, as explained in the “Special Report on Water” in the 20 May 2010 issue of The Economist. Fresh water is one of the most fundamental needs of humans and other organisms; each human needs to consume a minimum of about two liters per day. The world also faces greater fresh water demands from farming and industrial processes.
The hazards posed by insufficient water supplies are particularly acute. A shortage of fresh water may lead to a variety of crises, including famine, disease, death, forced mass migration, cross-region conflict/war, and collapsed ecosystems. In spite of the criticality of the need for fresh water and the profound consequences of shortages, supplies of fresh water are particularly constrained. It is estimated that 97.5% of the water on Earth is salty, and about 70% of the remainder is locked up as ice (mostly in ice caps and glaciers), leaving only a fraction of all water on Earth as available fresh (non-saline) water.
Moreover, the Earth's water that is fresh and available is not evenly distributed. For example, heavily populated developing countries, such as India and China, have many regions that are subject to scarce supplies. Further still, the supply of fresh water is often seasonally inconsistent. Meanwhile, demands for fresh water are tightening across the globe. Reservoirs are drying up; aquifers are falling; rivers are dying; and glaciers and ice caps are retracting. Rising populations increase demand, as do shifts in farming and increased industrialization. Climate change poses even more threats in many regions. Consequently, the number of people facing water shortages is increasing. Naturally occurring fresh water, however, is typically confined to regional drainage basins; and transport of water is expensive and energy-intensive.
On the other hand, many of the existing processes for producing fresh water from seawater (or to a lesser degree, from brackish water) require massive amounts of energy. Reverse osmosis (RO) is currently the leading desalination technology, but it is energy intensive and still relatively inefficient due to the large pressures required to drive water through membranes and their tendency for fouling. In large-scale plants, the specific electricity required can be as low as 4 kWh/m3 at 30% recovery, compared to the theoretical minimum around 1 kWh/m3, though smaller-scale RO systems (e.g., aboard ships) have much worse efficiency.
Other known systems used in existing seawater desalination systems include thermal-energy-based multi-stage flash (MSF) distillation, which typically also is an energy- and capital-intensive process, and multi-effect distillation (MED). In MSF and MED systems, however, the maximum brine temperature and the maximum temperature of heat input are limited in order to avoid calcium sulphate precipitation which leads to the formation of hard scales on the heat transfer equipment.
Humidification-dehumidification (HDH) desalination systems include an evaporator and a condenser as their main components and use a carrier gas (e.g., air) to communicate energy between the heat source and the brine. In the evaporator, hot seawater comes in direct contact with dry air and this air becomes heated and humidified. In the condenser, the heated and humidified air is brought into (indirect) contact with cold seawater and gets dehumidified, producing pure water and dehumidified air. Some of the present inventors were also named as inventors on the following patent applications that include additional discussion of HDH processes for purifying water: U.S. Pat. No. 8,292,272 B2; and U.S. Pat. No. 8,252,092 B2.
Although the importance of using a low entropic heat source is known in thermal applications, such as power production, use of a high-temperature energy source has not been feasible for seawater desalination until now because of the calcium sulphate fouling problems. In the last decade, the top brine temperature (and correspondingly the heating steam temperature) of thermal desalination systems has been increased from 70-90° C. to 90-120° C. using water softening technology, such as nanofiltration and hybrid systems. Further increase in heating steam temperature (and hence, decrease in total entropy entering the system) using such methods is thought to be unfeasible, however, due to the fouling problems.